The Association of Canadian Mountain Guides demonstrate Fixed-Point belay systems in this video.

Fixed-point belay systems as demonstrated make no effort to equalise the loading on the two anchors, but the video also states several times that fixed-point anchors are only appropriate for solid gear: bolts or ice-screws.

When considering 'bomb-proof anchors' in rock (bollards, for example, that have significant ratings [upwards of 80kN]), using two anchors in this fashion might allow for more ideal placement of the belay point: directly on an anchor, rather than between two anchors. If a single anchor is extremely unlikely to fail, this seems reasonable; I'm happy belaying off a single bollard in our area.

What is different about ice climbing? Are ice-screws considered stronger than natural protection in traditional rock climbing?
What are the advantages of ignoring equalisation and creating fixed-point belay systems?

Just because the anchor is bomb proof does not mean that the other points in the system (slings, carabiners, knots, etc.) are also bomb proof. An anchor is only as strong as the weakest point in the system. It's easy to not screw a screw gate completely shut, etc. which is why you should always have a backup regardless. It accounts for human error as well as failure.
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LiamMay 8 '14 at 8:28

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@Liam Completely agree - when using a single bomb proof anchor, redundancy is still as important. Tying multiple times to the same bomb-proof anchor accounts for human error, as well as sling or carabiner failure.
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simontMay 8 '14 at 11:54

What does "fixed point" mean? What would a non-fixed-point belay be?
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Ben CrowellMay 8 '14 at 21:33

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@BenCrowell 'fixed point' refers to the possible movement of the belay device. In a normal climbing anchor (equalised off three points), the belay device is attached to the power point, which can move. The video linked in the question puts the belay device directly onto one of the anchors, severely limiting the movement of the device ("fixing" it). However, this comes at a cost: you can't equalise such a system. Towards the end of the video a belayers movement during a climbers fall is shown; fixed-point seems to be advantageous compared to belaying from the belayers harness.
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simontMay 9 '14 at 2:06

1 Answer
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Ice screws have a tendency for pressure melting if constantly loaded over some time. Therefore if you build your fixed point belay system and and keep it under more or less constant load while belaying your partner, the screw might start to migrate as the ice below it melts slowly and refreezes above the screw. By this, a screw that was close to bomb-proof when you installed it might lose some of its strength over time.

If you build now a belay on two equalised screws, both get loaded and therefore both are affected by pressure melting, which means your whole anchor system might fail in case of a fall. This can to some point be avoided by building your belay point in a way that only one screw is loaded while having a second one for backup that will only get loaded if the first one fails.

Of course, one has to take care that the other points of the SERENE or EARNEST scheme are fulfilled – even more in this case. That means especially, that you have to ensure not to shock load the backup screw. Therefore take care that the backup screw is preferably in direct line with the expected load direction behind the loaded one and that they are connected without any slack webbing.